The present application relates to a cathode active material and a nonaqueous electrolyte secondary battery, particularly to a cathode active material, a cathode, and a nonaqueous electrolyte secondary battery, which are excellent in high capacity and high-power characteristics.
Recently, a lot of portable electronic devices such as camera-integrated videotape recorders (VTRs), cellular phones, or laptop computers has appeared and it is contemplated to reduce the size and weight thereof. Research and development of batteries, particularly secondary batteries to be used as portable power supplies of such electronic devices have been actively proceeding in order to improve their energy density.
Among batteries using a nonaqueous electrolyte, a lithium-ion secondary battery has been highly expected and the market for the battery has been growing since a greater energy density is obtained as compared to that of a lead battery which is an aqueous system electrolytic solution secondary battery in the past and a nickel-cadmium battery.
Since characteristics of the lithium-ion secondary battery such as lightweight and high energy density are suitable for application to electrical vehicles and hybrid electrical vehicles, examinations aimed at increasing the size of the battery and achieving a high power discharging capacity of the battery have been increased, particularly, in recent years.
With reference to a nonaqueous system secondary battery typified by the lithium-ion secondary battery, oxide cathodes such as LiCoO2, LiNiO2, and LiMn2O4 are generally used as the cathode active material. This is because a high capacity as well as a high voltage can be given and the oxide cathodes are excellent in high filling properties, which is advantageous for reduction in the size and weight of portable devices.
However, when these cathodes are heated in the charged state, they begin to release oxygen at 200 to 300° C. When the oxygen release starts, the battery can exhibit thermal runaway because a combustible organic electrolytic solution is used as an electrolytic solution. Therefore, when the oxide cathode is used, it is not easy to ensure the safety particularly in large-sized batteries.
On the other hand, as for a cathode material having olivine structure, oxygen release does not occur even when it exceeds 350° C. and the material is excellent in safety, which is described in A. K. Padhi, et al, J. Electrochem. Soc., Vol. 144, and p. 1188. As an example of the cathode material, lithium iron phosphate which is made mostly from iron (LiFe1-xMxPO4, wherein M is at least one of metallic materials selected from the group including manganese (Mn), nickel (Ni), and cobalt (Co)) is listed.
With reference to the cathode material having olivine structure, electric potential flatness is very high since charging and discharging are proceeded in a state where layers of LiFePO4 and FePO4 coexist. Therefore, when constant-current/constant-voltage charging (i.e., an ordinary charge mode for lithium ion battery) is performed, the charging is carried out in the constant-current charging state. Therefore, when the battery using the cathode material having olivine structure is charged at the same charging rate, the charging time can be reduced as compared to cathode materials in the past such as LiCoO2, LiNiO2, and LiMn2O4.
On the other hand, as for the cathode material having olivine structure, insertion and desorption of lithium during charging and discharging of the battery is slow and the electrical resistance is large as compared to lithium cobaltate (LiCoO2) used in the past. Overvoltage is increased during large current charging and discharging, which causes difficulty in obtaining sufficient charge-discharge capacities.
Efforts on the problems have been made. For example, a technique that conductive fine particles are supported on the particle surface of lithium iron phosphate and an active material is improved to improve charge-discharge capacities during large current charging and discharging is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 2001-110414 and JP-A No. 2003-36889.
Generally, in order to reduce the electrical resistance of the cathode, the cathode material having olivine structure is generally mixed with powder carbon such as carbon black, flake carbon such as graphite, and fibrous carbon.
Further, the method that a cathode active material having a sufficiently large specific surface area obtained by setting the particle diameter of primary particles of lithium iron phosphate to 3.1 μm or less is used to improve the electron conductivity in the cathode has been disclosed in JP-A No. 2002-110162.
Furthermore, a technique that binding of the cathode active material to the conductive agent, binding of the cathode active material to the cathode current collector, and binding of the cathode current collector to the conductive agent are improved by using a binder having a high binding capacity and load characteristics are improved during large current charging and discharging is disclosed in JP-A No. 2005-251554.
In fact, the electrical resistance of the cathode is reduced by the above-described techniques. However, considering application of the techniques to electrical vehicles and hybrid vehicles, the power characteristics are still insufficient as compared to lithium cobaltate having a layer structure or lithium manganate having a spinel structure, which remains as a big problem.
In order to obtain high-power characteristics using a composite metal material having olivine structure which is highly safe characteristics, for example, a method that the specific surface area is increased to make the reaction area larger by decreasing the particle diameter of an olivine-type cathode active material and secondary particles in which such primary particles are aggregated are formed, which are used as the cathode material has been proposed.
In this regard, the formation of secondary particles is performed to reduce the amount of the binder to be used. When the primary particles whose particle diameter is smaller are used as the cathode material, for example, power characteristics are improved by adding carbon black with a large specific surface area. The necessary amount of the binder is increased to stabilize a cathode mixture slurry to be produced when a cathode active material layer is formed or obtain adhesive strength of the electrode and a current collector foil. This causes problems of inhibition of the electrical conductivity of the electrode, reduction of high-power characteristics, and reduction of the productivity in a coating step because of an increase in the amount of solvent in the cathode mixture. Further, the amount of the cathode active materials is decreased, which leads the reduction of the battery capacity. In order to solve these problems, it is necessary to reduce the amount of the binder in the cathode mixture.
A cathode active material having a large particle diameter while maintaining the specific surface area can be produced by using secondary particles prepared from primary particles of the composite metal material having olivine structure whose particle diameter is small, which results in reduction of the amount of the binder.